<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        header {
            width: 100px;
            height: 50px;
            background-color: coral;
            margin: 0 auto;
        }
        
        div {
            width: 100px;
            height: 80px;
            background-color: cadetblue;
        }
        
        .one {
            margin-bottom: 10px;
        }
    </style>
</head>

<body>
    <header>

    </header>
    <div class="one">a</div>
    <div class="two">b</div>
</body>

</html>